The most commonly employed photographic elements are those which contain a radiation sensitive silver halide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation sensitive silver halide microcyrstals, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
Silver iodide, silver bromide, silver chloride and crystals consisting of mixtures of these halides are used as silver halides in photographic materials. Crystal structures for these halides range from so-called regular grains such as cubic, tetradecahedral, octahedral, rhombic dodecahedral, etc., to irregular grains such as tabular grains and rod-shaped grains.
Rod-shaped grains can translate into a number of advantages over conventional shaped grains, such as, for example, cubic grains. For example, in comparing two grains having the same grain volume, rod-shaped grains offer more surface area than a comparable volume of cubic grains. An increased surface area translates into potential photographic speed improvements. Further, we can expect better covering power for rod-shaped grains, and for comparable surface area grains, a rod-shaped grain would be expected to provide better granularity (i.e., less graininess). In short, a grain shape which exhibits greater surface area may offer faster speed, better covering power, and an improvement in granularity.
It is known to produce solid rod-shaped grains by utilizing certain growth modifiers during precipitation. U.S. Pat. No. 4,946,772 to Ogawa, for example, discloses the use of aminoazaindene compounds, such as guanine and adenine, as growth modifiers to produce solid rod-shaped silver chloride or silver chlorobromide emulsion grains. Such grains have a relatively low amount of surface per grain.
If one could produce rod-shaped grains whose inside was hollow, the surface area per unit grain would increase significantly. Generally, by increasing the surface area to volume ratio of the grains, the photographic speed and covering power increase, and less granularity is generally realized. The ability of developing solutions to dissolve the silver halide grain also increases as surface area of the grain increases, thereby beneficially effecting processing. Further, the annular opening at the ends of such hollow, rod-shaped grains would permit developer solutions, etc., to flow through the grain thereby further beneficially effecting processing. Hollow rod-shaped grain would also provide an interesting structure for capturing light, which could lead to interesting interference phenomena and other optical properties, which in turn could positively effect photographic performance. Also, compared to solid grains, hollow grains would present a greater amount of silver halide for exposure for equivalent weights of silver halide. This would translate into a cost savings due to the lower amount of silver needed to provide an adequate outside surface area of silver halide grains.